For example, Japanese Unexamined Patent Application Publication No. 2000-128009 (hereinafter, referred to as document '009) discloses a device that determines a tuning angle of wheels in accordance with a steering angle of a steering wheel which is mechanically separated from the wheels, as a vehicle steering device. In the vehicle steering device described in the document '009, a steering control unit of the vehicle steering device reads a current vehicle speed from a vehicle speed sensor when a start switch such as an ignition key is turned OFF during driving of a vehicle, and determines whether or not the current vehicle speed is lower than a predetermined speed. If the steering control unit determines that the current vehicle speed is not lower than the predetermined speed, a power supply of the vehicle steering device is not turned OFF. The steering control unit continuously reads the current vehicle speed from the vehicle speed sensor thereafter, and if the steering control unit determines that the current vehicle speed is lower than the predetermined speed, the steering control unit waits for a predetermined time and turns OFF the power supply of the vehicle steering device. The document '009 describes that, with the vehicle steering device, an occupant can continuously steer the vehicle until the vehicle speed becomes lower than the predetermined speed, for example, even when the start switch is unintentionally turned OFF during driving.
Also, the document '009 discloses a configuration in which a capacitor with a large capacitance is connected in parallel to a series circuit including a start switch and a vehicle-mounted battery as another embodiment of the vehicle steering device. With the description in the document '009, in the other embodiment, the capacitor is charged while the start switch is ON, and the power charged in the capacitor is discharged when the start switch is turned OFF and is supplied to the steering control unit. Hence, the document '009 describes that, in the other embodiment of the vehicle steering device, the occupant can continuously steer the vehicle while a discharge voltage of the capacitor is within an operable voltage range of the vehicle steering device, for example, even when the start switch is unintentionally turned OFF during driving.
Meanwhile, the vehicle speed detected by the vehicle speed sensor is also used for other purposes, such as a meter panel or a navigation device, in addition to the vehicle steering device. Hence, the vehicle speed sensor may not be directly connected to the vehicle steering device, and may output a vehicle speed signal through an in-vehicle network such as a control area network (CAN). Then, even if the power supply of the vehicle steering device is not turned OFF when the start switch such as the ignition key is turned OFF, a power supply of a device other than the vehicle steering device is turned OFF. Hence, electric power of the vehicle-mounted battery is not supplied to the vehicle speed sensor or an electronic control unit (ECU) to which the vehicle speed sensor is connected. Therefore, the steering control unit of the vehicle steering device described in the document '009 can no longer read the current vehicle speed from the vehicle speed sensor after the start switch is turned OFF.
Also, in the other embodiment of the vehicle steering device described in the document '009, the occupant can continuously steer the vehicle while the discharge voltage of the capacitor is within the operable voltage range of the vehicle steering device regardless of the current vehicle speed when the start switch such as the ignition key is turned OFF. Hence, the occupant can continuously steer the vehicle for a short time when the discharge voltage is low. Alternatively, it is required to increase the capacitance of the capacitor.